The desire to produce lower weight automobiles has led to the use of increased amounts of aluminum alloys in powertrain and body components. This usage is now extending to high magnesium content, aluminum sheet alloys that are capable of undergoing high elongation and substantial deformation into automobile body panels of complex shape. These aluminum alloys have a suitable composition and metallurgical microstructure for “superplastic forming” (SPF) on stretch form tooling at elevated forming temperatures. Aluminum Alloy 5083 is an example of a SPF sheet metal alloy that is now stretch formed at temperatures in the range of, e.g., 450 to 500° C. to form one piece vehicle deck lid panels, tailgate panels, door panels, quarter panels, and the like.
In an assembled vehicle, exterior panels have to be painted or otherwise decoratively finished. Of course, aluminum body panels can be spray painted because spray painting is the state-of-the-art automobile industry practice for producing commercially acceptable decorative finishes, sometimes termed Class-A finishes, for all types of substrates. However, there is practical interest in developing other finishing processes for aluminum alloy panels and other components. A known possibility is to anodize the surface of the aluminum panel and then decorate it by a coloring method. Anodizing of some aluminum alloys has been practiced for many years.
Anodizing is an electrochemical process in which an aluminum alloy part is made the positive electrode (anode) in an acid electrolyte (e.g., sulfuric acid) and a voltage is applied to establish the desired polarization to establish oxygen at the surface. This electrochemical process thickens and toughens the naturally occurring oxide and the resulting aluminum oxide substance is very hard.
In the electrochemical process the aluminum surface reacts with oxygen to produce adherent, oxide coatings:2Al+5H2O→Al2O3+O2↑+5H2↑
In the sulfuric acid anodizing process, the oxide is slowly dissolved by the electrolyte and a porous oxide coating is produced. The net coating growth rate and its porosity depends on the equilibrium set up between the film growth and dissolution. Typical anodized oxide thicknesses are in the range of five to thirty micrometers (μm) and a typical pore diameter is about twenty nanometers (nm). The porous structure allows secondary infusions such as organic and inorganic coloring, lubricity aids and the like.
Thus, the color anodizing of aluminum alloys is also a known art. But the results vary with the composition of the alloys. Alloying elements in the aluminum sheet affect the color of its anodized coating and the ability to achieve commercially acceptable finishes. For example, aluminum alloys containing more than about two to three percent by weight of magnesium tend to form dark gray anodized coatings by known anodizing processes.
A typical composition of AA5083 is, by weight, 4.60% magnesium, 0.79% manganese, 0.10% silicon, 0.02% copper, 0.18% iron, 0.01% zinc, 0.11% chromium, 0.01% titanium and the balance aluminum. This alloy composition, together with special thermo-mechanical processing of the sheet permits it to be processed by SPF into complex and durable body panel configurations. But the high magnesium content yields a gray, often dark, anodized finish by known anodizing practices. Furthermore the anodizing process results in a rough and low gloss surface. Despite repeated efforts, it is found that the anodized layer on an AA5083 sheet cannot be colored by known practices to yield commercially acceptable exterior panels for the automobile industry.
The literature of the prior art confirms this experience. For example, the text, “The Surface Treatment and Finishing of Aluminum and Its Alloys”, Wernick, Pinner and Sheasby, 1987, describes the effects of various alloying elements on the appearance of anodized commercial aluminum alloys. This text states that magnesium containing, aluminum alloys can give clear colorless anodized coating for magnesium up to 3% by weight. U.S. Pat. No. 4,601,796 entitled High Reflectance Semi-Specular Anodized Aluminum Alloy Product and Method of Forming Same, Powers and Dang, describes a method for obtaining a clear anodized layer for magnesium contents of only 0.25 to 1.5 weight percent magnesium.
However, it would now be very useful to produce clear anodized aluminum oxide coatings on aluminum alloys containing more than three percent by weight magnesium. Such coatings could be colored or finished by some other process to make automotive panels and other useful articles. Accordingly, it is an object of this invention to provide a process for forming such coatings and products. It is a more specific object of this invention to devise a method of producing a clear and glossy anodized aluminum oxide layer on a high magnesium content, aluminum alloy material of the type used in automotive vehicle external panels. The need is to provide such a coating that can be then provided with a colored or clear finish acceptable for commercial vehicle use (i.e., a Class A finish).